The present invention relates to nanocrystalline materials. More specifically, mesoporous nanocrystalline titanium dioxide materials comprising titanium dioxide and a second metal oxide are disclosed.
Since the discovery that titanium dioxide can act as a photocatalyst for the splitting of water, the substance has attracted the attention of scientists. The substance, however, exhibits two principal physical limitations. First, titanium dioxide absorbs light at energies greater than 3.2 eV; well outside the most intense region of the ambient solar spectrum (centered at xcx9c2.6 eV). Second, since titanium dioxide functions as a heterogeneous catalyst, catalytic activity is limited by surface area.
Titanium dioxide exists in at least three crystalline forms: anatase, rutile, and brookite. Anatase is the form that exhibits the highest catalytic activity and much effort has been directed toward providing anatase powders with increased stability and high surface areas.
Anatase nanocrystallites (i.e crystals with a diameter in the range of 20 xc3x85 to 100 xc3x85) are of interest because their photophysical and catalytic properties differ from the bulk material (See for example, Brus, J. Phys. Chem., 90: 2555-2560, 1986). Nanocrystallite properties are a direct result of the particle size and dimensionality, making adjustment of crystallite size and architecture an avenue to materials with novel photophysical and catalytic properties. Unfortunately, such small particles are difficult to handle, exhibit poor thermal stability, and exhibit a blue shift (i.e., further away from the ambient solar maximum) in their absorption relative to the bulk material.
Mesoporous materials offer an attractive alternative for increasing the surface area of a substance without making it difficult to handle. Mesopores (i.e. pores from about 20 xc3x85 to about 140 xc3x85 in diameter) provide a high surface area per unit mass through an increase in internal surface area and make it unnecessary to reduce the overall size of the particles to increase surface area. In contrast to microporous materials (i.e. materials having pore sizes of less than about 15 xc3x85), mesoporous materials show much higher rates of diffusion into and out of the pores, an attractive feature for a catalyst.
A general approach to the production of mesoporous materials by templating the formation of an inorganic oxide framework around surfactant micelles is disclosed in Huo et al., Nature, 368:317-321, 1994. Micelle size (a function of surfactant size) determines mesopore size in the as-synthesized materials. The surfactant micelles are removed from the resulting material by solvent extraction or thermal oxidation (calcination). The result is a mesoporous material having inorganic oxide walls between the pores.
Mesoporous silica and aluminosilicate materials with surface areas above 1000 m2 gxe2x88x921 have been synthesized by surfactant templating (see for example, Kresge et al., Nature, 359: 710-712 and Beck et al. J. Am. Chem. Soc., 114: 10834-10843). Mesoporous titanium doped metal silicates formed in a similar manner are disclosed in Hasenzahl, et al., U.S. Pat. No. 5,919,430. Thermally stable mesoporous materials with metal oxides as the principal wall component have been more elusive.
Mesoporous titanium dioxide materials are disclosed by Zhang in U.S. Pat. No. 5,718,878 (Zhang). These materials are formed using alkylamine micelles as the structure-directing agent. Zhang also discloses a method of treating the materials with a second metal compound after mesopore formation and wall crystallization has occurred. Despite this treatment, however, these materials still experience a significant loss of surface area upon calcination.
A mesoporous titanium dioxide material that does not lose pore structure upon calcination is described by Elder et al. (Elder et al., Chem. Mater., 10: 3140-3145, 1998). This material, comprising nanocrystalline anatase particles surrounded by amorphous zirconium oxide is stable and exhibits high surface areas. However, as is typical of nanocrystalline materials in general, the material exhibits a blue shift in photoabsorption energy (PE), exacerbating one of the principal limitations of titanium dioxide materials, insufficient absorption of solar radiation.
Mesoporous nanocrystalline titanium dioxide heterojunction materials as described herein are a surprising new class of materials that overcome the principal limitation of zirconium oxide stabilized mesoporous nanocrystalline titanium dioxide.